Frequency responsive circuit



y 1946- R. c. SANDERS. JR 2,403,615

FREQUENCY RESPONSIVE CIRCUIT Filed Nov. 30, 1942 F/90M HUD/0 1907 Bnventor Dovo-en CfianDe /JQ:

(Ittorneg Patented July 9, 194a FREQUENCY RESPONSIVE CIRCUIT Royden C. Sanders, Jr., Hightstown, N. J., assignor of America, a corporation to Radio Corporation of Delaware Application November 30, 1942, Serial No. 467,445

3 Claims. -(Cl.- 175-4320) This invention reiates to frequency responsive circuits, and more. particularly to a system for operating a relatively rugged relay in response to signals of predetermined or selected frequencies. when the input is above a preselected frequency, the relay is operated to one position; when the input is below this frequency the relay i operated to another position. Thus the device may be used to control motors or indicators in response to changes inthe frequency of an alternating current source of relatively low power capability.

One application of this system is in the control of aircraft by means of a radio altimeter of the type described on pages 86-90 of Civil Aeronautics Bulletin No. 29, published by the Civil Aeronautics Administration. The radio altimeter provides an audio frequency output directly proportional in frequency to the height of the airplane above the ground. The airplane may be controlled to fly at some selected level by operating motors connected to the elevator surfacesin response to variations in the output frequency of the altimeter from the frequency corresponding to the selected level. A frequency responsive relay system for this purpose must withstand the vibration and acceleration forces occurring in flight without failure or false operation.

Accordingly, it is one object of this invention to provide an improved method of and means for controlling a relativelyheavy and insensitive relay in response to variations from a predetermined value in the frequency of an alternating voltage.

Another object is to provide an improved device which is uniformly sensitive to frequency variations, independent of the selected frequency.

A further object is to provide animproved method .of and means for controlling the damping of a relay or other electromagnetic device, to prevent erratic operation in response to sudden variations of the control voltage or received signal.

These and other objects will become apparent to those skilled in the art upon consideration of the following description. with reference to the accompanying drawing, which is a schematic circuit diagram of an embodiment of the invention.

A pentode amplifier tube i is provided, with a grid leak 3, a self bias resistor I, and a load resistor 1. The plate of the tube I is coupled through a capacitor 9 to a double diode II. The plate I! of one diode section and the cathode I! of the other diode section are connected to the capacitor I. The cathode II of the first section is grounded, and the anode I! of the other section is connected to ground through to a capacitor 2|. The grid 23 of an amplifier tube 25 is connected to the diode plate 19 through a resistor 21. A load resistor 28 is'connected from the anode IQ to the arm 3| of a tap switch 33. A plurality of resistors 35 are connected in series with an adjustable resistor 31 across a direct current source-38. The taps 38 of the switch 33 are connected between the resistors 35 so that an adjustable portion of the direct current voltage may be applied to the load resistor 29. A relay ll is con=- nected in the anode circuit of the tube 25.

The device operates as iollows:

Audio frequency voltage applied across the resistor I is amplified in the tube l. The amplitude of the input voltage is made high enough to cut 03 the tube on the negative peaks and cause saturation on the positive peaks. This clips the peaks of the amplified voltage, producing an output of rectangular wave form. The time constant of the condenser 8 and the resistor i is made so small that the condenser 9 will be almost 109% discharged or charged in one half cycle of the highest frequency at which this device is to operate.

The positive half cycle of the rectangular wave causes the condenser 9 to be charged through the diode section I3, H. The negative half cycle of the rectangular wave discharges the condenser Si through the diode section i5, 59 into the condenser 2| and hence stores its discharge current in the condenser 2|. The condenser 2i discharges through the load resistor 29, potentiometer 3i and the direct current source 38. The condenser e charges and discharges through a voltage range Esw equal to the peak'to peak amplitude of the rectangular wave voltage across the resistor '5 minus any average negative voltage Ef developed across the condenser 2|. Hence every cycle, (Esw-t-Ej) C. Coulombs or (Esw+E.f) (C) (F) amperes will be passed by the diode section 15, i9, where F is the frequency of the applied signal.

and C is the capacitance of the capacitor H. H

Eb is the positive bias potential developed between point SI and ground, and R. is the resistance or the resistor 29, then E!=Eb (EswieEf) (C) (F) (R) when E! is negative only. It is seen therefore that the voltage E! is the resultant some at the positiveibias voltage Eb from point 3| to ground and the negative voltage developed across the re sister 2! due to the current (Esw-l-Ef) (C) (F). when E! is negative, the above relations hold exactly. However if the current (Esw+El) (C) (F) is not sufiicient to make El negative, then enough steady D.-C. current will flow thru the diodes II, II and II, I! to maintain E! at approximately sero volts. Thus El will not vary appreciably with frequency until the current is large enough to cause E! to become negative. I

square wave voltage Esw. The voltage at the Eb tap ll is adjusted to a value correspondingtothe frequency at which the relay is to beoperated.

. when the frequency is below the selected value, the grid 23 of the tube II is positive and the relay is energized. maintaining the contacts 43 in their upper positions. When the voltage!!! becomes slightly greater in magnitude than the voltage at the tap II, the grid 23 becomes negative and the anode current of the tube 2| decreases, reducing the energization of the relay II and causing the 4 quency responsive circuit for operating a relay in to input of a predetermined frequency. a unidirectional voltage, related in magnitude to the frequency of the input, is derived. An adiustable unidirectional voltage is opposed to the frequency derived voltage, and the resulting diiference voltage is used to control an amplifier tube connected to the relay.

1 claim as my invention:

1. A frequency responsive relay system comprising a limiter, a rectifier coupled to the output of said limiter through a capacitor, a second capacitor in series with said rectifier, a discharge path for said second capacitor including a direct current source connected to oppose the voltage produced across said second capacitor by the current flowing through said rectifier, a second rectifier connected across said second capackor and said first rectifier and poled oppositely thereto, an amplifier tube with-its control grid coupled to said discharge circuit, a relay connected in the anode circuit of said amplifier tube, and means for adjusting the voltage of said direct current source which is-applied to said discharge circuit, whereby said voltage overcomes the volt age produced across said second capacitor at a predetermined frequency or input to said limiter.

movable contacts 4! to go to their lower positions.

constant equal toor greater than that of the I capacitor 2i and resistor 2!. This causes the anode current of the tube 25 to change at the same rate.with a decrease in frequency as 'it does with an increase in frequency, preventing erraticopcrationoftherelayll.

The invention has been described as a fre- 2. The method of operating a relay in response to variations in the frequency of an alternating voltage comprising the steps of deriving a unidirectional voltage related in magnitude to said frequency, opposing said voltage to a second unidirectional voltage of adjustable magnitude, controlling a power circuit in accordance with the amount by which said second voltage exceeds said directional voltage related in magnitude to said. frequency, opposing said voltage to a second unidirectional voltage of adjustable magnitude, controlling, a direct current in accordance with in amount by which said second voltage exceeds said first voltage, and energizing said relay with said current. a

- ROYDEN C. SANDERS, Js. 

